1. Field of the Invention
The present invention relates to a radio-frequency receiver unit for a magnetic resonance apparatus of the type having a reception antenna that is connected to an input of a noise-matched field-effect transistor preamplifier with a low input impedance.
2. Description of the Prior Art
A radio-frequency receiver unit of the general type described above is known from U.S. Pat. No. 4,825,162. Several radio-frequency receiver units of this sort are used as an array system in imaging with magnetic resonance, in order to detect a magnetic resonance signal simultaneously with several reception coils. Even though such local antennas have the disadvantage of a high signal-to-noise ratio, this arrangement allows their use be used even given large areas of examination. In order to reduce unwanted couplings between the individual reception antennas in the reception mode, the reception antennas are damped by the input impedance of the preamplifiers respectively coupled thereto.
In German PS 44 22 069, a reception antenna for a magnetic resonance apparatus is described that can be detuned by means of a circuit inserted in the antenna conductor.
German OS 38 06 371 describes the problem that a high antenna impedance is the cause of a high noise level.
It would be advantageous if the region of sensitivity (field of view FOV) of the antenna array were additionally alterable, in order to evaluate only signals of particular radio-frequency receiver units, as warranted. The remaining radio-frequency units would then be inactive. This possibility would be of particular interest for all local receiver coils, if an overview image (body survey or scan) is first intended to be produced using a whole-body antenna. Subsequently, high-resolution images (though with only a limited imaging area) could then be produced with local antennas for the actual diagnosis. All reception antennas could then remain physically in the magnetic resonance apparatus, so that a repositioning of the patient during an examination would no longer required be, even if different body areas must be examined successively (e.g., head, cervical and thoracic spinal column or pelvis, upper and lower limbs). It is known to detune local antennas that are used as pure reception antennas during the excitation of the magnetic resonance signals so as to effectively deactivate those antennas. This takes place, for example, by disconnecting the reception antennas in high-ohmic fashion by a switched parallel resonance circuit.
By detuning reception antennas that are switched inactive during the reception-mode, the coupling between the active or receiving and inactive or non-receiving antennas would simultaneously be minimized. A disadvantage of detuning, however is, that the preamplifiers of the inactive radio-frequency reception units respectively emit a very strong noise signal at their outputs. This noise couples to the reception antennas of the active radio-frequency receive units, as well as to the whole-body antenna in case of a body survey, and degrades the signal to noise ratio. If in the above examples the degraded image quality is unacceptable, a possible remedy would be to completely shield the signal line and preamplifier, to prevent coupling. This solution, however, is expensive, and is not always possible for reasons of space.